1. Field of the Invention
The present invention relates to an illuminating device using a bar-shaped light guide and a contact-type image sensor incorporated with the illuminating device.
2. Description of the Prior Art
A contact-type image sensor is used as a device for reading a document in a facsimile machine, a copying machine, an image scanner device and the like. The contact-type image sensor is provided with a line-illuminating device for linearly illuminating a document surface along the main scanning zone.
A line-illuminating device using a light guide is known. For example, Japanese Patent No. 3083092 discloses a line-illuminating device in which a light-emitting unit is provided on one end of the light guide and a light-scattering pattern is formed along the longitudinal direction of the light guide to spread and reflect the light emitted from the light-emitting unit. A plurality of light sources (i.e., LEDs) of the light-emitting unit is provided on a normal line of the light-scattering pattern, wherein a light-emitting surface of the light guide is formed in a different direction from the normal line.
In the prior art including the above-mentioned line-illuminating device, the light guide is made rectangular in its cross-section. In the case where the light-scattering pattern is formed on the light guide by printing, it is formed on the side face including long sides in the longitudinal direction for easy printing. In the embodiment of Japanese Patent No. 3083092, the side face including long sides in the longitudinal direction is provided with a slanting notch on which the light-scattering pattern is printed.
The light guide is obtained by injection-molding a resin such as acrylic. When the light guide of a rectangular cross-section is obtained by injection molding, a surface sink is mainly produced during molding on the side face including the long sides.
As described above, in the light guide having the rectangular cross-section, the light-scattering pattern has been printed on the side face including the long sides in the longitudinal direction, in other words, on the side where the surface sink is produced in the prior art. In this manner, a pattern shape is different from the intended shape and as a result, the intensity of illuminating light in a main scanning direction may be uneven.
In the case where the notch is provided to print the light-scattering pattern as described in the embodiment of Japanese Patent No. 3083092, it is troublesome because printing itself is effected on a surface which is not parallel with the side face of the light guide. Further, uneven intensity of illuminating light is readily obtained because a surface sink is easily produced on that surface.
In the conventional line-illuminating device, the light-emitting surface is comparatively small in area and the direction of illuminating light emitted from the light guide is precisely described. As a result, attenuation of light intensity in the light emitting direction becomes large and light distribution characteristics (i.e., the ratio of optical power in a predetermined position to optical power in a position out of the predetermined position) become worse. For example, fluctuation of output power in the case where a document is slightly elevated in the contact-type image sensor becomes large. In this manner, severe positioning accuracy is required when the line-illuminating device is incorporated with the contact-type image sensor.